I don't believe there's any way to have those changes flow into pre-existing dashboards without manually going in and having the admins make the changes. However, anyone with publish priviledges should be able to publish those templates for others to use, including site admins.
Thank you for your reply.
I have read at site:Tableau Friction: Make Dashboards' formatting consistent. and in there they say " The tool: SetDashboardFormatAll.rb is available on GitHub here, or can be copy/pasted from below."
That tool seems to have the functionality we are looking for. Would Tableau allow us to use GitHub code?